Sulfuric acid, which is a very important starting material for the chemical industry, usually is produced by the so-called double absorption process as it is described for example in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A25, pages 635 to 700. Sulfur dioxide (SO2) is converted to sulfur trioxide (SO3) in a converter by means of a catalyst. The sulfur trioxide obtained is withdrawn after the converter and supplied to an intermediate absorber or a final absorber (e.g. hot absorber), in which the gas containing sulfur trioxide is guided in counterflow to concentrated sulfuric acid (H2SO4) and absorbed in the same. The resulting highly concentrated sulfuric acid is partly withdrawn as product and upon dilution with water partly recirculated to the absorber for renewed absorption.
To adjust the acid concentration for the absorber circuit, apparatuses as shown in FIG. 1 have been used so far. In these apparatuses, concentrated sulfuric acid is supplied through a supply conduit via an inlet to a mixing chamber which substantially is disposed at right angles to the supply conduit. Into the end adjacent to the inlet a lance with nozzle openings (so-called “clarinet”) extends, through which water is charged to the sulfuric acid stream for adjusting the concentration, i.e. for dilution. In the mixing chamber concentrated sulfuric acid and water are mixed and the concentration-adjusted sulfuric acid is removed through a discharge conduit via an outlet at the end opposite to the supply conduit. For intermixing concentrated sulfuric acid and water, static mixers can also be incorporated in the mixing chamber. However, the flow cross-section is reduced thereby, which leads to a pressure loss. Despite the comparatively large amount of equipment, no uniform concentration can be achieved at the entrance to the outlet conduit. As seen over the cross-section of the conduit, the concentration deviations are about 0.5 wt-%.